1. Field of the Invention
The present invention relates to a heater power controller for controlling the temperature of the ink in the print head of an ink jet printer.
2. Description of the Prior Art
An example of a conventional heater power controller of this kind is shown in FIG. 1. In the drawing, a heater resistor 1 is for heating the ink in the ink chamber in a print head for an ink jet printer. The heater resistor 1 is adapted to be driven (supplied with a current) by means of a driver 2. On the other hand, the driver 2 is energized by means of an ON-OFF control signal delivered from a comparator 3. A temperature detector 4 is adapted to detect the temperature of the ink to be heated. The comparator 3 makes a comparison between the output (a voltage signal corresponding to the ink temperature) of the temperature detector 4 and the output (a voltage corresponding to a predetermined ink temperature) of a reference voltage generator 5 and delivers an ON signal ("1" signal) when the former is smaller than the latter, i.e., when the ink temperature has not risen to the predetermined temperature, and an OFF signal ("0" signal) when the former is larger than the latter to the contrary.
Such a problem, however, has been encountered when the heater resistor is made by printing a print resistor on a head base that the stability of print resistor is lowered due to the materials thereof and the resistance value thereof is varied widely such as from 20.OMEGA. to 100.OMEGA., for example. Accordingly, even if a constant voltage is applied on the heater resistor the heating temperature of the heater resistor is varied, thereby causing the head to be overheated, for example. Specifically, if the electric power applied on the heater is excessive the control ability of the heater temperature becomes wrong as shown in FIG. 2(a). The heater temperature is not stabilized at a setting temperature .theta..sub.0 in cooperation with the hysteresis in the control system or the transmission delay from the heater to the sensor, so that a large overshoot is generated. Accordingly, the ink temperature in the nozzle becomes unsteady. The fluctuation (ripple) in temperature should be suppressed as shown in FIG. 2(b).
FIG. 3 is a graph showing a relation between the ink viscosity and the temperature. In the graph, the abscissa shows the temperature, the ordinate shows the viscosity of ink, and .epsilon..sub.0 shows the ink viscosity at the temperature .theta..sub.0. The viscosity of ink affects on the property of ink jet from the nozzle. If the temperature of nozzle is varied as shown in FIG. 2(a), the viscosity of ink is varied in the large extent at the value of .epsilon..sub.0, so that the speed of ink droplets becomes unstable extremely, and if it is varied as shown in FIG. 2(b), the speed becomes stable.
Further, if the temperature of head is varied as shown in FIG. 2(a), the ink oozes out of the nozzle hole of head on the nozzle surface, because the ink is larger in coefficient of heat expansion than the head base plate. Specifically, if the temperature is elevated the ink is expanded and oozes out of the nozzle and if the temperature is lowered the ink is sucked into the nozzle and such phenomenon may be repeated. Large quantity of oozed ink causes the printing paper arranged in the viscosity of the nozzle surface to be soiled. Even if the printing is being carried out by jetting the droplets in the state that the ink oozes out, the oozed droplets in front of the nozzle prevent the droplet to be flown and air bubble are sucked into the nozzle, so that the reliability becomes lowered.
In such a conventional apparatus, accordingly, to cope with variations in resistance of the heater resistor 1, the dummy resistor 6 is connected in series to the heater resistor 1 to regulate the resistance value of the heater resistor 1 as shown in FIG. 4, thereby to consume unnecessary power in the dummy resistor and to drive the heater resistor 1 with a constant power. In this case, a dummy resistor usable to this object becomes very large in size and high in cost and many kinds of dummy resistors must be prepared according to the value of heater resistor, thereby causing the printer complicated in construction. In addition, the power consumed in the dummy resistor becomes wasteful heat losses and the temperature in the printer case becomes high, disadvantageously.